Utility power producers use combined cycle systems because of their inherent high efficiencies and installed cost advantage. Combined cycle systems typically include a gas turbine, heat recovery steam generators, generators, controls, and a steam turbine. Combined cycle systems use the hot exhaust gas from gas turbines to make steam, which drives a steam turbine. The combination of a gas turbine and a steam turbine achieves greater efficiency than would be possible independently. The output of a combined cycle system is affected by altitude and variations in the ambient temperature.
Various methods are available for improving the performance of combined-cycle power plants. Improvements can be made in plant output or efficiency beyond those achievable through higher steam temperatures; multiple steam-pressure levels or reheat cycles. For example, it has become commonplace to install gas fuel heating on new combined-cycle power plants to improve plant efficiency. Additionally, gas turbine inlet air cooling is sometimes considered for increasing gas turbine and combined-cycle output. Another approach is supercharging (compressor inlet pressurization). Supercharging of a gas turbine entails the addition of a fan to boost the pressure of the air entering the inlet of the compressor. Typically, in the case of supercharging, the additional stage of compression is not driven by the main gas turbine shaft, but rather by an electric motor. In some cases, the parasitic power of the fan motor is more than the additional output of the gas turbine, so the net result is a capacity loss.